1. Field of the Invention
The present invention relates to a semiconductor optical device with a buried heterostructure (BH) useful for optical communication and to the fabrication method of the same. More particularly, the present invention relates to a buried structure semiconductor optical device structured such that a Ru-doped semi-insulating layer is inserted between a mesa stripe and a burying layer.
2. Description of the Related Art
As the burying layer of a semiconductor optical device such as a semiconductor laser diode or an optical modulator and the like, a current blocking layer formed by a pn buried structure and a current blocking layer formed by a semi-insulating layer are known. According to these current blocking layers, for example, currents can be concentrated on a light-emitting region in a semiconductor laser diode.
Since parasitic capacitance of the current blocking layer formed by the pn buried structure is larger than that of the current blocking layer formed by the semi-insulating layer, it is difficult to realize high-speed operation for the devices with the pn buried structure.
As for a current blocking layer using Fe-doped indium phosphide (InP) as a semi-insulating film, when a Zn-doped InP layer is used as a p type cladding layer, the conductivity of the current blocking layer near the mesa stripe is changed to p type due to inter-diffusion between Fe in the current blocking layer and Zn in the p type cladding layer, so that there occurs a problem in that resistivity of the current blocking layer becomes low. As a result, leakage current and junction capacitance increase. These problems also cause degradation of device characteristics.
That is, inter-diffusion between iron (Fe) and zinc (Zn) occurs when a Fe doped burying layer is placed adjacent to a Zn-doped cladding layer and/or a Zn-doped contact layer. The inter-diffusion causes degradation of device characteristics, especially, modulation characteristics. In addition, Zn atoms moved in an interstitial site due to inter-diffusion diffuse not only to the burying layer but also to an active layer adjacent to the Zn-doped cladding layer (in the case of the semiconductor laser diode), or to a photoabsorption layer adjacent to the Zn-doped cladding layer (in the case of the optical modulator). Thus, there is also a problem in that light emitting efficiency of the active layer is lowered, or the extinction characteristic of the photoabsorption layer is degraded.
Conventionally, there are the following technologies to solve these problems. Japanese laid-open patent application No. 10-22579 discloses a semiconductor laser diode having nondoped InAlAs as the burying layer. That is, since Fe is not doped in the burying layer, inter-diffusion between Fe and the p type dopant does not occur, so that degradation of characteristics due to inter-diffusion does not occur. However, since InAlAs is nondoped, there is a problem in that resistivity of InAlAs is low.
In addition, Japanese laid-open patent application No. 9-214045 discloses that a Fe diffusion preventing layer is inserted between a Zn-doped cladding layer and a Fe-doped InP burying layer. That is, as shown in FIG. 1, the Fe diffusion preventing layer 16 is inserted between the Fe-doped InP burying layer 17 and the mesa stripe which is formed by a buffer layer 12, an active layer 13, a cladding layer 14 and a contact layer 15. In the Japanese laid-open patent application No. 9-214045, as a specific example of the Fe diffusion preventing layer 16, an n-InP layer and a Fe-doped InP layer are disclosed, in which vacancy concentration of the Fe-doped InP layer is equal to or more than 5.0xc3x971014 cmxe2x88x923.
However, in order to grow the Fe-doped InP layer of which vacancy concentration is equal to or more than 5.0xc3x971014 cmxe2x88x923 as the Fe diffusion preventing layer 16, it is necessary to use a higher growth temperature (660xc2x0 C.) than that used for growing the usual Fe-doped InP layer. Thus, thermal degradation may occur on the sides of the mesa stripe during growth.
In addition, although diffusion of Fe can be prevented by inserting an n-InP layer as the Fe diffusion preventing layer 16, there is a problem in that leakage currents increase since resistivity of the n-type InP layer between the cladding layer and the burying layer is low.
In addition, Japanese laid-open patent application No. 61-290790 discloses that a burying layer of Fe-doped InAlAs is formed by liquid phase epitaxy. Also in this case, as mentioned above, there is a problem in that Znxe2x80x94Fe inter-diffusion occurs between the Zn-doped cladding layer and the Fe-doped InAlAs burying layer.
Recently, it was found that Ru-doped semi-insulating layer rarely causes inter-diffusion between Ru and Zn. Thus, a buried structure laser diode using a Ru-doped InP layer which is a semi-insulating film as the current blocking layer is proposed in A. van Geelen et al., Appl. Physics Letters 73, No 26 pp 3878-3880 (1998), and A. van Geelen et al., 11th International Conference on Indium Phosphide and Related materials TuBl-2 (1999) for example. FIG. 2 shows the configuration.
However, as for the Ru-doped InP burying layer proposed in the above-mentioned documents, a precipitate of Ruxe2x80x94P is apt to occur. Thus, there is a problem in that it becomes difficult that Ru effectively acts as the semi-insulating dopant of InP.
Therefore, in order to suppress occurrence of the Ruxe2x80x94P precipitate, it is necessary to grow the semi-insulating layer under very restricted conditions such as under lowered growth pressure, or lowering the supplying amount of phosphine (PH3), which is the source material for P, or under low growth temperature of about 580xc2x0 C. or the like.
In patent DE19747996C1, when the number of hydrogen groups of the group V precursor is equal to or less than 2, the growth temperature of the Ru-doped compound semiconductor can be lower than that for PH3 or AsH3 with 3 hydrogen groups, which is mainly used. Since the decrease in the number of hydrogen groups reduces the decomposition temperature of the group V precursor, the growth temperature can be lowered, so that occurrence of the precipitate with Ru can be suppressed.
When growing the semi-insulating layer at the low growth temperature as mentioned above, there occurs a problem in that a defect such as hillock is apt to occur on the surface of the burying layer.
In addition, as for the Ru-doped InP, the surface of the crystal becomes very sensitive, and poor crystal habit easily occurs. Thus, depending on the condition of the surface layer after performing RIE (Reactive Ion Etching) or wet etching, a void may occur in the Ru-doped InP burying layer 30 as shown in FIG. 3. In FIG. 3, the reference numeral 10 indicates an n-InP substrate, 20 indicates the semiconductor stacked body, 21a indicates an n-InP cladding layer, 22a indicates an active region formed by a MQW active layer or MQW photoabsorption layer, 23a indicates a p-InP cladding layer, 24a indicates a p-InGaAsP contact layer, 25a indicates a p-InGaAs contact layer, 31a indicates a void in the side wall of the mesa stripe, and 31b indicates a void in the side wall including InAlAs.
In addition, when burying a device with the active region formed by an InAlAsxe2x80x94InGaAlAs multiple quantum well layer that acts as the active layer or the photoabsorption layer, a void easily occurs on the side wall of the active region. Thus, there is a problem of reliability, reproducibility and the like. In addition, it is difficult to change a physical constant such as the lattice constant and the index of refraction for the InP layer and the like.
There is a method of mass transport as a method for burying the side surface of the active region as disclosed in Japanese laid-open patent application No. 8-250806, for example.
Processing damage on the side surface of the active region due to formation of the mesa stripe is removed by using wet etching. After that, the device is loaded in a growth reactor. When the growth temperature rises, a part of the cladding layer is dissolved, is moved to the side surface of the active region and recrystallized. As a result, the side surface of the active region is buried by a material for the cladding region.
According to this method, surface damage of the processed layer due to dry etching can be removed, and thermal damage due to the rise in temperature can be prevented.
However, by this mass transport method, since the part of the material of the cladding region is dissolved, moved to the side surface of the active region and recrystallized for performing the burying process, impurity doped in the cladding layer is moved with the material and included in the burying layer that is recrystallized. Therefore, the area of junction region may increase and leakage currents may increase.
In addition, the impurity included in the material of the dissolved cladding layer diffuses to the active region adjacent to the burying layer, and the diffusion may cause deterioration of light emitting characteristics of the active layer or extinction characteristics of the photoabsorption layer. Especially, when Zn, which is a p-type impurity, is included in the region formed by the mass transport, the p-type burying layer and the region formed by mass transport are connected in the case of pn-junction buried structure, or, it causes inter-diffusion between Fe and Zn, and leakage currents and junction capacitance increase when using a Fe-doped semi-insulating layer as the burying layer.
As mentioned above, a Fe-doped semiconductor crystal is used for the semi-insulating buried heterostructure (SIBH). However, there is a problem in that inter-diffusion between Fe in the burying layer and Zn, which is a dopant of the p-cladding layer and p-contact layer, occurs at the interface of the burying layer and the Zn-doped layer. As a result, Zn atoms diffuse to the burying layer, which causes device characteristics degradation, especially, modulation characteristics degradation.
In addition, in a conventional technology using a Ru-doped InP burying layer such as A. van Geelen et al., Appl. Physics Letters 73, No 26 pp 3878-3880 (1998), bis (xcex75-2,4-dimethylpentadienyl ruthenium (II)) is used as a source material gas of Ru, and an InP crystal in which Ru is doped to 4xc3x971013 cm3 is grown by using a metalorganic vapor phase epitaxy method. The mass transport is not used.
In a conventional technology disclosed in A. van Geelen et al., 11th International Conference on Indium Phosphide and Related materials TuBl-2 (1999), fabrication of a semiconductor laser diode is disclosed in which a Ru-doped semi-insulating InP layer and an n-InP hole blocking layer formed on the Ru-doped InP layer are used as the burying layer. Also in this example, growth of the burying layer is performed by an epitaxial growth method using the MOVPE method, and mass transport is not used.
That is, when using mass transport in the conventional technology, there are problems in that the region formed by mass transport becomes p type, junction capacitance increases, and leakage current paths occur. Therefore, performance of the device degrades, and there is a problem in that fabrication yield is low. In addition, Ru doping is performed by the epitaxial growth method and mass transport is not used.
An object of the present invention is to provide a semiconductor optical device having a structure in which Zn atoms do not diffuse to the burying layer and to the active region.
Especially, in a semiconductor optical device with a structure in which a diffusion preventing layer is inserted between the mesa stripe and the semi-insulating burying layer, the object of the present invention is to provide a semiconductor optical device with a structure in which the material of the diffusion preventing layer is improved, so that thermal degradation of the side walls of the mesa stripe is prevented during growth of the burying layer and leakage current paths of the side walls of the mesa stripe are eliminated.
Another object of the present invention is to solve drawbacks of the conventional fabrication method of a semiconductor optical device using mass transport. That is, the object is to provide a method for fabricating a high performance semiconductor optical device that prevents leakage current paths and inter-diffusion, and that has few leakage currents and small junction capacitance and provides improvement in yield.
The object can be achieved by a semiconductor optical device, comprising:
a semiconductor substrate;
a stacked body formed at least by a cladding layer having a first conductivity, an active region formed by an active layer or a photoabsorption layer and a cladding layer having a second conductivity, the stacked body being provided on the semiconductor substrate and formed like a mesa stripe;
wherein both sides of the stacked body are buried by a burying layer formed at least by a semi-insulating semiconductor crystal;
the burying layer includes a first layer that is placed adjacent to both sides of the stacked body and a second layer that is placed adjacent to the first layer;
the first layer includes Ru as a dopant;
composition of the second layer is different from the composition of the first layer, or, a dopant of the second layer is different from a dopant of the first layer.
In the present invention, the semiconductor optical device is formed such that the Ru-doped layer is provided in both sides of the stacked body, and at least a layer is provided in which the composition of the layer is different from the composition of the Ru-doped layer or a semi-insulating dopant of the layer is different from a semi-insulating dopant of the Ru-doped layer.
Conventionally, as disclosed in A. van Geelen et Al., 11th IPRM TuBl-2 (1999), a single burying layer is used. On the other hand, in the present invention, at least two different layers are used as the burying layer. A first layer in the two different layers is a relatively thin Ru-doped layer, and is placed adjacent to the mesa stripe. A second layer in the two different layers is a relatively thick layer provided such that it covers the first layer, in which the composition of the second layer is different from that of the first layer, or, the dopant in the second layer is different from that in the first layer.
In a fabrication process, the growth of the Ru-doped compound semiconductor layer is more difficult than that of the Fe-doped compound semiconductor layer. However, by adopting the buried structure of the present invention, the ratio of growth process of the Ru-doped layer to total burying layer growth process can be decreased. Since the Ru-doped layer is sensitive to surface condition of a processed substrate, it is difficult to grow the Ru-doped layer. However, by adopting the structure of the present invention, the difficulty for growth can be relieved since the thickness of the Ru-doped layer can be relatively thin. In addition, since the major part of the burying layers is formed by a stable burying layer, fabrication yield can be improved.
Therefore, a device structure can be provided in which defect due to difficulty of growth can be prevented, and fabrication yield can be improved. That is, Zn diffusion can be prevented by using the Ru-doped compound semiconductor layer, and in addition, defects occurring in the fabrication process can be prevented.
In the above structure, the composition of the first layer may be Ru-doped InGaAlAs or Ru-doped InAlAs.
By adopting such composition, since the dopant is Ru, inter-diffusion between Ru and Zn can be prevented. In addition, as for an alloy semiconductor crystal layer such as InAlAs and InGaAlAs, since the diffusion rate of a doped impurity is much smaller than that of a binary compound semiconductor such as InP, the inter-diffusion between Ru and Zn can be further suppressed compared with the case of InP. Therefore, resistivity is not lowered, and a highly resistive current blocking layer can be obtained. Thus, increases of capacitance and leakage currents can be suppressed.
In addition, the dependence of the quality of the Ru-doped InAlAs or Ru-doped InGaAlAs on surface condition is smaller than that of the Ru-doped InP, and the Ru-doped InAlAs or Ru-doped InGaAlAs is easier to grow than the Ru-doped InP. Thus, defects hardly occur.
In addition, the Ru-doped InAlAs or InGaAlAs is not susceptible to surface damage or an oxidized layer due to RIE or wet etching and the like. Thus, voids hardly occur in the Ru-doped InAlAs or InGaAlAs compared with Ru-doped InP.
In addition, since the binding strength between doped Ru and As forming InAlAs and InGaAlAs is much weaker than that between Ru and P forming InP, the possibility that a precipitate such as Ruxe2x80x94As occurs during the growth of crystals of InAlAs or InGaAlAs becomes small. Therefore, it becomes possible to easily grow a highly resistive semiconductor layer without adopting very narrow growth conditions as in the conventional Ru-doped InP.
In addition, since the InAlAs and the InGaAlAs are alloy semiconductor crystals, it becomes possible to easily change a physical constant such as refractive index and bandgap and the like while keeping the condition of lattice matching with the InP substrate by changing the composition of Ga and Al.
In addition, since the physical constants such as refractive index and bandgap can be changed by changing the composition, flexibility of device design increases.
That is, according to the present invention, since leakage currents can be suppressed and voids hardly occur, it becomes possible to provide a reliable semiconductor optical device with high performance.
In addition, since Al is not exposed on the surface of the semiconductor stacked layers due to burying another burying layer on the top surface of InAlAs or InGaAlAs, it becomes possible to easily form other devices on the surface of the semiconductor stacked layers.
Since the first layer of the present invention comprises the Ru-doped InAlAs or InGaAlAs, the first layer is different from nondoped InAlAs layer disclosed in Japanese laid-open patent application No. 10-22579, the Fe-doped InAlAs layer disclosed in Japanese laid-open patent application No. 61-290790 and the Ru-doped InP layer disclosed in A. van Geelen et al.
That is, the present invention is different from Japanese laid-open patent application No. 10-22579 in that Ru is doped so that resistivity is increased. In addition, this invention is different from Japanese laid-open patent application No. 61-290790 in that Ru is used as the semi-insulating impurity that is doped instead of Fe. Therefore, diffusion of Zn dopant in the cladding layer does not occur. In addition, the present invention is different from A. van Geelen et al. in that Ru is doped in InAlAs or InGaAlAs not in InP. Therefore, physical constants such as refractive index or bandgap and the like can be changed by changing the composition, so that flexibility in device design is increased.
In addition, the present invention is different from Japanese laid-open patent application No. 9-214045 because the present invention adopts a structure in which another burying layer is stacked on, for example, a top surface of the burying layer formed by the Ru-doped InAlAs or InGaAlAs.
That is, in the technology in Japanese laid-open patent application No. 9-214045, the Fe diffusion preventing layer formed by a low-resistive n-InP layer is inserted between the Zn-doped cladding layer and the Fe-doped InP burying layer. On the other hand, according to the present invention, a burying layer formed by highly resistive Ru-doped InAlAs or InGaAlAs is inserted between the cladding layer and other burying layer. Therefore, leakage currents do not increase. Ru-doped InP can be used as the second layer in the above structure.
In the above-mentioned structure, Ru-doped InP can be used instead of Ru-doped InAlAs or Ru-doped InGaAlAs as the material of the first layer. Accordingly, Zn diffusion can be prevented and the increase in leakage current which was the conventional problem can be prevented. In addition, Fe-doped InP can be used as the second layer. By adopting this structure, fabrication technology established by the conventional Fe-doped InP can be used for growth of the second layer, which occupies most of the burying process.
More concretely, in the semiconductor optical device, since the Ru-doped layer is inserted between the mesa stripe and the Fe-doped InP layer, a semi-insulating impurity does not diffuse from the Fe-doped InP layer to the mesa stripe.
Therefore, resistivity of the Fe-doped InP layer is not lowered, and an impurity such as Zn atoms doped in a layer forming mesa stripe does not diffuse to the Fe-doped InP layer or the active region. In addition, since Ru-doped InP is semi-insulating, leakage current paths do not occur between the mesa stripe and the Fe-doped InP burying layer. In addition, since the growth temperature of the Ru-doped compound semiconductor layer is the same or lower than that of Fe-doped InP, thermal degradation of side walls of the mesa stripe can be suppressed during growth.
In the semiconductor laser diode disclosed in Japanese laid-open patent application No. 9-214045, a Fe diffusion preventing layer formed by an n-InP or Fe-doped InP layer whose vacancy concentration is equal to or more than 5.0xc3x971014 cmxe2x88x923 is inserted between the mesa stripe and the Fe-doped InP burying layer. On the other hand, a different point is that, according to the semiconductor optical device of the present invention, a Ru-doped layer is inserted.
Therefore, the Ru-doped layer acts as a diffusion preventing layer and a current blocking layer. Thus, leakage current paths do not occur in the side walls of the mesa stripe. That is, as for effects, the present invention is different from the conventional technology in that the Ru-doped layer acts as the diffusion preventing layer, and in addition, acts as the current blocking layer.
In addition, Japanese laid-open patent application No. 9-214045 discloses only the effect of preventing diffusion. Although it discloses that a Fe-doped InP layer in which vacancy concentration is equal to or more than 5.0xc2x71014 cmxe2x88x923 is used as a Fe diffusion preventing layer, it does not disclose that the Fe-doped InP layer indicates high resistivity and becomes a current blocking layer. In addition, it is impossible to think that it indicates enough high resistivity because the defect density is too high since the vacancy concentration is equal to or more than 5.0xc3x971014 cmxe2x88x923.
In addition, the present invention provides a structure in which Ru-doped InP formed by mass transport is provided in the both sides of the active region for preventing Zn diffusion and increase of current leakage.
That is, the present invention is a semiconductor optical device comprising:
a semiconductor substrate;
a stacked body formed at least by a cladding layer having a first conductivity, an active region formed by an active layer or a photoabsorption layer and a cladding layer having a second conductivity, the stacked body being provided on the semiconductor substrate and formed like a mesa stripe;
wherein both sides of the stacked body are buried by a burying layer formed at least by a semi-insulating semiconductor crystal;
the width of the active region is smaller than the width of the cladding layers of the stacked body; and
a Ru-doped semi-insulating layer is provided in a space between the burying layer and the active region in both sides of the active region. The Ru-doped semi-insulating layer can be Ru-doped InP formed by using mass transport. In addition, a Ru-doped semi-insulating layer can be provided as the burying layer by epitaxial growth method such that the Ru-doped semi-insulating layer covers the Ru-doped semi-insulating layer provided in the space. In addition, composition of the Ru-doped semi-insulating layer provided by the epitaxial growth method may be Ru-doped InP or Ru-doped InAlAs or Ru-doped InGaAlAs.
In addition, the object can also be achieved by a method used for fabricating a semiconductor optical device by using mass transport, the method comprising the steps of:
forming a stacked body by successively growing at least a cladding layer having a first conductivity, an active region formed by an photoabsorption layer or an active layer, and a cladding layer having a second conductivity;
forming a mask with a predetermined shape, and etching the stacked body by using the mask, so that a mesa stripe is formed;
etching both sides of the active region by performing selective etching such that the width of the active region becomes smaller than the width of the cladding layers in the stacked body;
burying both sides of the active region by mass transport while supplying a source material gas of Ru; and
burying both sides of the stacked body with a Ru-doped semi-insulating semiconductor.
According to the present invention, the Ru-doped semiconductor layer is semi-insulating, and inter-diffusion between Ru and p-type impurity does not occur, so that stable highly resistive layer can be realized and a current blocking layer with good quality can be realized. Thus, by providing the Ru-doped layer in both sides of the active region, the pn junction area can be decreased and leakage currents in the side walls of the active region can be decreased, so that a high-performance modulator and light emitting device with good characteristics can be realized.
Ru can be easily doped in a region formed by mass transport by supplying a Ru precursor into the growth reactor during the mass transport process. Therefore, by burying the both sides of the active region by using mass transport, the buried region (region formed by mass transport) becomes semi-insulating. Thus, by compensation of holes by Ru, a semi-insulating layer can be formed even when a p-impurity (Zn) is included in the region formed by mass transport. The present invention is different from the conventional technology in that the metalorganic gas including Ru is supplied into the growth reactor during the mass transport process.